1. Field of the Invention
The present invention relates to improvements in processes and apparatus for producing synthesis gas, or syngas, from light hydrocarbons such as methane or natural gas by the partial oxidation thereof. Such syngas, comprising a mixture of carbon monoxide and hydrogen, is useful for the preparation of a variety of other valuable chemical compounds, such as by application of the Fischer-Tropsch process.
The combustion stoichiometry of methane gas at 1000.degree. F. is highly exothermic and produces CO.sub.2 and H.sub.2 O according to the following reaction: ##EQU1## The formed gases are not useful for the production of valuable chemical compounds, and the high temperatures generated present problems with respect to reactors and catalysts which would be required to produce valuable products from the formed gases.
It is known to produce useful gases, known as synthesis gases or syngases, by partial oxidation of methane and other light hydrocarbon gases, by steam or CO.sub.2 reforming of methane and other light hydrocarbon gases, or by some combination of these two chemistries. The partial oxidation reaction of methane is a less highly exothermic reaction which, depending upon the relative proportions of the methane and oxygen and the reaction conditions, can proceed according to the following stoichiometry: ##EQU2##
It is most desirable to enable the partial oxidation reaction to proceed according to the latter reaction in order to produce the most valuable syngas and minimize the amount of heat produced, thereby protecting the apparatus and the catalyst bed, and to reduce the formation of steam, thereby increasing the yield of hydrogen and carbon monoxide, and enabling the steam-reforming reaction to convert any steam and hydrogen into useful syngas components.
Conventional syngas-generating processes include gas phase partial oxidation process (GPOX), the autothermal reforming process (ATR), the fluid bed syngas generation process (FBSG), the catalytic partial oxidation process (CPO) and various processes for steam reforming. Each of these processes has advantages and disadvantages when compared to each other.
The ATR process and the FBSG process involve a combination of gas phase partial oxidation and steam reforming chemistry.
In the ATR process, illustrated, for example, by U.S. Pat. No. 5,492,649 and Canadian Application 2,153,304, the gases are intended to react before they reach the catalyst, i.e., the oxidation chemistry occurs in the gas phase, and only the steam reforming chemistry occurs in the catalytic bed. In fact, long residence times are required because diffusion flames are initiated with a large amount of over-oxidation, accompanied by a large heat release. Thus, time is required for the relatively slow, endothermic gas phase steam reforming reactions to cool the gas enough for introduction into the catalyst bed to prevent thermal damage to the catalyst.
In the FBSG process illustrated for example by U.S. Pat. Nos. 4,877,550; 5,143,647 and 5,160,456, the hydrocarbon gas, such as methane,and oxygen or an oxygen-supplying gas are introduced separately into a catalyst fluid bed for mixing therewithin. While the gases may be introduced at a plurality of sites, to more evenly distribute the gases over the inlet of the fluid bed of the reactor, the fact that the gases mix within the fluid bed results in over-oxidation hot spots and catalyst sintering or agglomeration due to the oxygen concentration being higher and closer to full-combustion stoichiometry in areas closest to the oxygen injection sites. The gas phase partial oxidation and steam reforming chemistry employed in the FBSG and the Autothermal Reforming (ATR) processes have very similar material balance when using similar feed. However, ATR is limited in size by the scaleability of its injector design, and the more-scaleable FBSG is economically debited by the cost of fluid solids and dust cleanup and by the expense of replacing agglomerated and/or eroded catalyst. The dust comprises catalyst fines due to catalyst attrition in the bed, and these fines are expensive to clean out of the syngas. While the chemistry is correct, these two processes have significant drawbacks. Both require very large reactors. For FBSG there is a significant expense in fluid solids management. For Autothermal Reforming there is a large and problematic methane/oxygen feed nozzle.
Fluid bed processes are well known for the advantages they provide in heat and mass transfer characteristics. Such processes allow for substantially isothermal reactor conditions, and are usually effective in eliminating temperature runaways or hot spots. However, with O.sub.2 injection the complete elimination of hot spots is impossible, although the fluid bed does tend to minimize the heat intensity. Catalyst strength or attrition resistance is important for maintaining the integrity of the catalyst and minimizing the formation of fine particles that may be lost from the fluidized bed system, especially those particles not recoverable by use of cyclones and deposited in down stream equipment causing fouling or reverse reactions as temperature is decreased. Erosivity or the tendency to erode equipment must be contained since catalyst erosivity usually increases in catalysts with increased attrition resistance.
Additionally, the relatively high temperatures, e.g., above about 1650.degree. F., found in reforming reactions, where oxygen gas is present can cause agglomeration of the fluidized catalyst particles, leading to lower catalytic efficiency (e.g. lower conversion), larger particles that are more difficult to fluidize, greater wear on equipment due to greater momentum and impact forces, and clogging of lines. For example, a common catalytic material nickel, even when deposited in small amounts on a suitable carrier e.g., less than about 5 wt % nickel on a support, tends to soften at reaction temperatures (due to its reactivity with the support phase and formation of reactive/lower melting mono- and polymetalic oxide phases), which become sticky, and generally lead to particle agglomeration. Particle agglomeration, in fact, tends to increase as the amount of nickel present in the catalyst bed increases or as the Ni-containing phase is subjected to multiple oxidizing and reducing cycles as it is transported through the fluid bed.
Particle agglomeration is particularly increased by the high temperature, e.g., above about 2500.degree. F., that occur when oxygen is introduced separately into the fluid bed. Thus, maintaining the level of nickel on catalyst at rather low levels, and avoiding exposure of the catalyst to high temperatures, minimizes particle agglomeration. On the other hand, sufficient nickel is required for providing economical feed conversions to synthesis gas, i.e., within about 250.degree. F. approach to equilibrium, thereby minimizing the level of CH.sub.4 exiting the syngas generation zone. It is a principal object of the present invention to provide an improved FBSG process which avoids the aforementioned disadvantages of conventional FBSG processes by operating under conditions which avoid high runaway oxidation temperatures and catalyst particle agglomeration, erosion, and dust formation, and enable the use of more active catalysts due to the reduction in severity of temperature and reduction-oxidation cycling experienced by the catalyst.